1. Field of the Invention
The present invention relates to antenna arrangements for suppressing selected sidelobes and, more particularly, to antenna arrangements comprising a main antenna including a main reflector and a feedhorn, and sidelobe suppressing means comprising either (a) at least two auxiliary antennas each including a reflector having a predetermined aperture and a feedhorn for launching the same signal, with a proper level and phase, as the main antenna, the auxiliary antennas being disposed on opposite sides of the main antenna with spacings that approximately equal the edges of the aperture of the main antenna in the plane of the sidelobes to be suppressed, or (b) sections of the reflective surface of the main reflector of the main antenna which are displaced by a predetermined amount in a direction parallel to the boresight axis of the main antenna in the plane of the sidelobes to be suppressed.
2. Description of the Prior Art
In radar systems and in terrestrial and satellite communication systems, various techniques have been used to reduce certain sidelobes and in turn the interference therefrom in adjacent links. In receiving systems, undesired sidelobe signals are generally suppressed by receiving the desired signal at a directional antenna and possible interfering signals at a separate omnidirectional antenna. The derived interfering signals are then used to cancel interference in the desired signal using various circuitry configurations. In this regard see, for example, U.S. Pat. Nos. 3,094,695 issued to D. M. Jahn on June 18, 1963 and 3,202,990 issued to P. W. Howells on Aug. 24, 1965.
Alternatively, for transmission purposes U.S. Pat. No. 3,704,464 issued to C. J. Drane, Jr. et al on Nov. 28, 1972 discloses a method for maximizing aerial directive gain while simultaneously placing nulls in the far-field radiation pattern of an array of N elements which are arbitrarily positioned. The patented method permits specification of directions of up to N-1 independent pattern nulls and/or sidelobes while assertedly providing maximum gain in some prescribed direction. This control is apparently achieved by varying only the amplitude and phase of the element currents in association with a standard gain formula.
U.S. Pat. No. 3,815,140 issued to W. E. Buehler et al on June 4, 1974 relates to a multiple feed arrangement for microwave parabolic antennas which include a parabolic reflector, and a plurality of individually fed illuminators. Each illuminator alone produces a beam of certain dimensions, and by combining the beams through the use of a predetermined configuration of illuminators, the physical configuration of the beam, including sidelobes, may be accurately controlled. Furthermore, certain illuminators may be fed by different information sources, thus resulting in a multiple information beam pattern.
Sidelobe suppression in directional antennas is usually accomplished by combining the signal from the main antenna with that of an auxiliary antenna, suitably adjusted in amplitude and phase. Since the auxiliary antenna is usually much smaller than the main antenna, their radiation patterns do not match. The result is that sidelobe suppression will be effective only over a narrow frequency band and angular sector. By introducing special equalizing networks, sidelobe suppression can be obtained over an increased bandwidth.
The problem remaining in the prior art is to be able to provide antenna arrangements which produce low sidelobes in selected directions to provide minimal interference in certain links over a wide bandwidth using simple techniques and without the need for equalization networks.